High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al., U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate) because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss and lower surface friction.
It has previously been disclosed by Phipps, Jr., and Wambach, U.S. Pat. No. 4,124,561, that mineral fillers, such as mica, talc or the like can make useful molding compositions with poly(1,4-butylene terephthalate), and that the addition of glass fibers to them improves shatter resistance. Phipps, Jr. and Wambach, U.S. Pat. No. 4,115,333, disclose further that the addition of zinc stearate improves the warp-resistance of compositions comprising poly(1,4-butylene terephthalate), glass and mineral filler, e.g., talc, clay, silica, calcium silicate, mica, and the like. Gasman and Charles, U.S. Pat. No. 4,111,893, disclose that mineral filled poly(1,4-butylene terephthalates) in general are difficult to produce with excellent mechanical properties and have proposed to overcome this by providing the filler, e.g., glass spheres, wollastonite, mica, feldspar, novaculite, kaolin, and the like, with a coating of a difficult to prepare and exotic hydrolysis product of a sulfonyl azido alkyl trimethoxy silane. The disclosures of the above-identified patents are incorporated herein by reference.
Surprisingly, it has now been discovered that poly(tetrafluoroethylene), a readily available resin, is admirably suited to coat mineral fillers prior to mixing them into poly(1,4-butylene terephthalate) resin, if care is taken not to expose the coated particles to sintering temperatures prior to mixing. As will be shown later, mica coated with poly(tetrafluoroethylene) under non-sintering conditions, and then added to poly(1,4-butylene terephthalate) markedly improves important properties in pieces molded from such compositions, particularly impact strength, deformation temperature under load (DTUL), and flexural strength.